1. Field of the Invention
This invention relates to a process for producing highly water absorptive polymers.
The polymers obtained by the process of this invention are highly absorptive not only to pure water but also to various electrolytic aqueous solutions such as a physiological saline solution and artificial urine. Moreover, they can absorb water very quickly and the resulting gelled polymers exhibit a high strength. Therefore, the highly water absorptive polymers obtained by the process according to the present invention are advantageously used for various absorbent materials, in particular, in the field of sanitary goods.
2. Background Art
In recent years, highly water absorptive polymers are practically used not only for sanitary goods such as sanitary napkins and paper diapers but also for materials to be used in industrial fields as well as in horticultural and agricultural fields, such as water holding materials, dropwise condensation preventing materials, freshness preserving materials and dehydrating materials. It is expected that the application of highly water absorptive polymers will broaden in the future.
Conventionally known highly water absorptive polymers are a hydrolyzate of a graft copolymer of starch and acrylonitrile, a crosslinked product of carboxymethyl cellulose, a crosslinked product of polyacrylic acid (salt), a copolymer of acrylic acid (salt) and vinyl alcohol, a crosslinked product of polyethylene oxide, and the like.
A highly water absorptive polymer is required to have a high water absorption capacity and to exhibit a high water absorption rate when it is brought into contact with an aqueous liquid. These properties are especially important to sanitary goods such as sanitary napkins and paper diapers, which have rapidly spread recently and are produced by rapidly-advancing techniques. However, none of the above-described conventional highly water absorptive polymers can satisfactorily meet the above requirements.
In general, the water absorption capacity of a highly water absorptive polymer is incompatible with the water absorption rate and the strength of the gelled polymer (hereinafter referred to simply as the gel strength); that is, both the water absorption rate and the gel strength of a highly water absorptive polymer decrease as its water absorption capacity is made higher. Therefore, how to harmonize these conflicting properties has been a most serious problem in the art. In particular, when an acrylic monomer as used in the present invention is employed as a starting material, the monomer itself tends to cause crosslinking (so-called self-crosslinking) during polymerization even in the absence of a crosslinking agent if a persulfate such as ammonium persulfate or potassium persulfate is used as an initiator. The self-crosslinking of an acrylic monomer generally provides a polymer which has a sufficiently high water absorption rate and gel strength, but has an insufficient water absorption capacity.
The self-crosslinking of the monomer may be suppressed, for example, by remarkably decreasing the monomer concentration, or by using an azo compound as a polymerization initiator, such as the aqueous solution polymerization method disclosed in Japanese Patent Kokai Publication No. 58-71907 and the surface treating method disclosed in Japanese Patent Kokai Publication No. 59-62665. However, the former method is not practical because its production efficiency is extremely low, and the latter method using an azo initiator is also unsatisfactory so far as the present inventors know.
It is, therefore, a primary object of the present invention to provide a process for producing, with high efficiency, a highly water absorptive acrylic polymer which has a remarkably enhanced water absorption capacity while having a high water absorption rate and a high gel strength.